<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <style>
    .outer {
      display: flex;
      height: 100px;
    }

    .left {
      width: 100px;
      background: tomato;
    }

    .right {
      width: 100px;
      background: gold;
    }

    .center {
      flex: 1;
      background: lightgreen;
    }
  </style>
  <body>
    <div class="outer">
      <div class="left"></div>
      <div class="center"></div>
      <div class="right"></div>
    </div>
  </body>
</html>
